This invention relates to gas turbine engines, and in particular to cooling of combustion chamber walls in such engines.
The combustion chambers in gas turbine engines are subject to very high temperatures in use and, as efforts are made to increase engine efficiency, higher operating temperatures become desirable. However, the ability of the combustion chamber walls to withstand higher temperatures becomes a limiting factor in engine development. New wall materials to withstand higher temperatures are constantly being developed, but there is usually some cost or functional penalty involved. As metal alloys become more exotic, they tend to be more expensive, both in the materials required and in the complexity of manufacture. Ceramic materials, on the other hand, while being able to withstand high temperatures, tend to exhibit low mechanical strength.
An alternative approach to the development of new materials is to improve the systems for cooling the walls in use. In one air cooling system, the combustion chamber is formed with twin walls spaced apart from each other by a small distance. Compressed air from the engine compressor surrounds the combustion chambers within the engine casing, and holes formed in the outer wall of the twin walls of the chamber allow air to impinge on the inner wall, creating a first cooling effect. Such holes are normally referred to as impingement holes. The air in the space between the walls is then admitted to the combustion chamber through a series of smaller holes, normally referred to as effusion holes, through the inner wall which are arranged to aid laminar flow of the cooling air in a film over the inner surface of the inner wall, cooling it and providing a protective layer from the combustion gases in the chamber. Examples of such cooling arrangements are disclosed in United Kingdom Patent No. A-2173891 and United Kingdom Patent No. A-2176274. This type of arrangement can have a significant effect in extending the operating life of a combustion chamber.
It has now been found that by adopting a particular arrangement of effusion holes and associated impingement holes, the cooling effect can be enhanced.
According to the invention, there is provided a combustion chamber for a gas turbine engine, the combustion chamber having:
upstream and downstream ends relative to the direction of combustion gas flow therethrough,
an inner wall,
an outer wall spaced apart from the inner wall thereby to define a cavity between the walls,
the outer wall having a plurality of impingement cooling holes therethrough, whereby, during operation of the engine, compressed air surrounding the chamber can pass through the impingement holes to impinge on the inner wall,
the inner wall having a plurality of effusion holes therethrough, whereby air can effuse from the cavity between the inner and outer walls into the combustion chamber, there being a greater number of effusion holes than impingement holes,
wherein the effusion holes are arranged in groups, each group comprising a plurality of effusion holes substantially equally spaced apart from each other around a central effusion hole, each group of effusion holes having an impingement hole located in the outer wall such that air passing through the impingement hole impinges on the inner wall at a predetermined position relative to the central effusion hole within a boundary defined by the group of diffusion holes.
Preferably, the effusion holes are arranged in groups of seven, comprising six effusion holes substantially equally spaced around a central seventh effusion hole. The predetermined position of the impingement hole relative to the central effusion hole is preferably such that air passing through the impingement hole impinges on the inner wall closer to the central effusion hole than to the other effusion holes and is in alignment with the central effusion hole along the direction of combustion gas flow in the chamber. Hence, each impingement hole may be located upstream or downstream of the central effusion hole in the group, but is more preferably arranged downstream of the central effusion hole such that the centerline of the impingement hole is spaced from the centerline of the central effusion hole by a distance at least equal to the diameter of the impingement hole.
The groups are suitably arranged in rows extending circumferentially of the chamber. For convenience in manufacturing and to ensure uniform airflows, each group may be spaced from the next in the row by a distance substantially equal to the spacing between adjacent holes in a group and the groups in any one row may be displaced circumferentially from those in the or each adjacent row by a distance substantially equal to half the distance between the central holes in adjacent groups in a row. Furthermore, the longitudinal spacing between the rows may be such that the distance between two adjacent effusion holes which belong to different groups in adjacent rows is the same as the distance between two adjacent holes in the same group of effusion holes.
In a preferred embodiment, additional effusion holes are provided centrally of each set of six holes defined between two adjacent groups in one row and the displaced adjacent group in the next row.
The relative sizes and numbers of the impingement holes and the effusion holes are preferably such that, during operation of the engine, the pressure differential across the outer wall is at least twice the pressure differential across the inner wall; for example, approximately 70% of the total pressure drop across the outer and inner walls may occur across the outer wall and the remainder across the inner wall.
It has been found that the combustion chamber wall temperature during operation of the engine is significantly lower using the arrangement of the invention than is achieved with known cooling arrangements. Benefits are gained from the enhanced film cooling not only in the combustion chamber can, but also into the transition duct which leads from the can into the turbine inlet. The enhanced cooling extends the life of the combustion chamber can and its transition duct, especially when combustion temperatures are increased to improve combustion efficiency.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.